The ability to obtain and indicate, in a useful manner, highly accurate information regarding the direction of motion and velocity of a movable member is very important in controlling tools, equipment and apparatus, monitoring processes, determining liquid level, and for many other purposes. Typically, due to the nature of the tools, equipment apparatus or process, the environment in which they operate and the need for continuous information, the determination of direction of motion and velocity is usually accomplished by a sensor installed proximately to the member whose motion is being sensed but out of contact therewith in order to avoid interfering with or influencing in any way the movement of the member. Moreover, the motion sensed will, most usually, be transmitted to a location remote from the movable member where it may be observed, recorded or used to control the operation of equipment or processes.
Permanent magnet sources and magnetic field sensors provide a favored combination for sensing the motion of machine parts. The advantages associated with this inherently non-contact mode of sensing are practically realizable in small, robust and low cost devices. Major applications involve the sensing of the speed and direction of rotating members. Common practice is to employ one or the other of the two general methods illustrated in FIG. 1. In FIG. 1(a), a circumferential region of the rotating "target" member is endowed with equally spaced, salient magnetic "poles," either by the attachment of discrete permanent magnets or by localized permanent magnetizations. In FIG. 1(b), rotation of a toothed, ferromagnetic target causes periodic alterations in the permeance of the magnetic circuit. The magnetic field sensor (FS) may be either a passive type, wherein an EMF is induced in a coil by the time varying magnetic field, or an active type, e.g., a Hall effect or magnetoresistive element which detects changes in magnetic field strength. Direction of motion is sensed from features of the signal or from the phase sequence of signals from two, circumferentially spaced field sensors. Speed is determined from the frequency of the periodic field sensor signal. In selecting field sensors, however, passive field sensors are particularly inappropriate for systems which require speed signals from zero rotation since such sensors are unable to measure rotation speed below a fixed threshold. For this reason, as well as the sensitivity of passive sensors to air gap changes and vibration, active sensors have become the sensors of choice.
Motion sensing methods and sensors which rely on the time interval between discrete events require that at least one such interval occur before motion can be unambiguously discerned. Moreover, rapid sensing of startup direction, large accelerations or small oscillatory motion requires closely and accurately spaced magnetized regions or physical notches on the target surface. Dealing with those requirements increases both the complexity and cost of the complete sensor.
It is, therefore, apparent that despite the availability of and advances in direction of motion and velocity sensing to date, there exists a need for a simple device for sensing both the speed and direction of motion of a target surface which requires neither teeth nor a periodic pattern of remanent magnetization; which is not time interval dependent; which is able to operate with a smooth target surface; and, which is economical, accurate and reliable.